After returning to his home institute in Kiev, Churyumov examined all the photographic plates more closely. On 22 October, about a month after the photograph was taken, he discovered that the object could not be Comas Solà, because it was about 1.8 degrees off the expected position. Further scrutiny produced a faint image of Comas Solà at its expected position on the plate, thus proving the other object to be a different body.[27]

The comet consists of two lobes connected by a narrower neck, with the larger lobe measuring about 4.1 km × 3.3 km × 1.8 km (2.5 mi × 2.1 mi × 1.1 mi) and the smaller one about 2.6 km × 2.3 km × 1.8 km (1.6 mi × 1.4 mi × 1.1 mi).[3] With each orbit the comet loses matter, as gas and dust are evaporated away by the sun. It is estimated that currently a layer with an average thickness of about 1 ± 0.5 m (3.3 ± 1.6 ft) is lost per orbit.[28] The comet has a mass of approximately 10 billion tonnes.[4]

The two-lobe shape of the comet is the result of a gentle, low-velocity collision of two objects. The "terraces", layers of the interior of the comet that have been exposed by partial stripping of outer layers during its existence, are oriented in different directions in the two lobes, indicating that two objects fused to form Churyumov–Gerasimenko.[29][30]

Dust and cosmic rays on the surface of the comet in 2016, with stars moving in the background

There are 26 distinct regions on Churyumov–Gerasimenko, with each named after an Egyptian deity; regions on the large lobe are named after gods, whereas those on the small lobe are named after goddesses. 19 regions were defined in the northern hemisphere prior to equinox.[31][32] Later, when the southern hemisphere became illuminated, seven more regions were identified using the same naming convention.[33][34]

Features described as gates, twin prominences on the surface so named for their appearance,[clarification needed] have received names by the Rosetta Science Working Team. They are named after deceased members of the Rosetta team.[35]

During Rosetta's lifetime, many changes were observed on the comet's surface, particularly when the comet was close to perihelion.[36][37][38] These changes included evolving patterns of circular shapes in smooth terrains that at some point grew in size by a few meters per day.[39][40] A fracture in the neck region was also observed to grow in size; boulders tens of meters wide were displaced, sometimes travelling more than 100 meters; and patches of the ground were removed to expose new features. A number of collapsing cliffs have also been observed. One notable example in December 2015 was captured by Rosetta's NAVCAM as a bright patch of light shining from the comet. Rosetta scientists determined that a large cliff had collapsed, making it the first landslide on a comet known to be associated with an outburst of activity.[41][42]

The orbit of 67P/Churyumov–Gerasimenko moves from just inside the orbit of Mars to just outside the orbit of Jupiter, seen here at perihelion in August 2015.

This animation consists of 86 images acquired by Rosetta's NavCam as it approached 67P in August 2014.

Like the other comets of the Jupiter family, Churyumov–Gerasimenko probably originated in the Kuiper belt and was ejected towards the interior of the Solar System, where later encounters with Jupiter successively changed its orbit.

Up to 1840, the comet's perihelion distance was 4 AU (600 million km), too far for the Sun to vaporize the nucleus. In 1840 Jupiter changed the orbit to a perihelion distance of 3 AU (450 million km), and later encounters further decreased that distance to 2.77 AU (414 million km).[43]

In February 1959, a close encounter with Jupiter[44] moved Churyumov–Gerasimenko's perihelion inward to about 1.29 AU (193 million km), where it remains today.[14][43]

Before Churyumov–Gerasimenko's perihelion passage in 2009, its rotational period was 12.76 hours. During this perihelion passage, it decreased to 12.4 hours, which likely happened because of sublimation-induced torque.[7]

As of September 2014[update], Churyumov–Gerasimenko's nucleus had an apparent magnitude of roughly 20.[13] It came to perihelion on 13 August 2015.[2][12] From December 2014 until September 2015, it had an elongation less than 45 degrees from the Sun.[45] On 10 February 2015, it went through solar conjunction when it was 5 degrees from the Sun and was 3.3 AU (490 million km) from Earth.[45] It crossed the celestial equator on 5 May 2015 and became easiest to see from the Northern Hemisphere.[45] Even right after perihelion when it was in the constellation of Gemini, it only brightened to about apparent magnitude 12, and required a telescope to be seen.[12] As of July 2016[update], the comet had a total magnitude of about 20.[13]

First image of the comet taken by Rosetta with Messier 107 in view, 21 March 2014

As preparation for the Rosetta mission, Hubble Space Telescope pictures taken on 12 March 2003 were closely analysed. An overall 3D model was constructed and computer-generated images were created.[46]

On 25 April 2012, the most detailed observations until that time were taken with the 2-metre Faulkes Telescope by N. Howes, G. Sostero and E. Guido while it was at its aphelion.[citation needed]

On 6 June 2014, water vapor was detected being released at a rate of roughly 1 L/s (0.26 USgal/s) when Rosetta was 360,000 km (220,000 mi) from Churyumov–Gerasimenko and 3.9 AU (580 million km) from the Sun.[47][48] On 14 July 2014, images taken by Rosetta showed that its nucleus is irregular in shape with two distinct lobes. Two explanations were proposed at the time: that its shape may have resulted from asymmetric erosion due to ice sublimating from its surface to leave behind its lobed shape. Newer evidence supports the contact binary model. The size of the nucleus was estimated to be 3.5×4 km (2.2×2.5 mi).[17][49][50]

Animation of Rosetta's orbit around 67P from 1 August 2014 to 31 March 2015Rosetta· 67P

Beginning in May 2014, Rosetta's velocity was reduced by 780 m/s (2,800 km/h; 1,700 mph) with a series of thruster firings.[17][51] Ground controllers rendezvoused Rosetta with Churyumov–Gerasimenko on 6 August 2014.[18][19] This was done by reducing Rosetta's relative velocity to 1 m/s (4 km/h; 2 mph). Rosetta entered orbit on 10 September, at about 30 km (19 mi) from the nucleus.[18][19][52]

Descent of a small lander occurred on 12 November 2014. Philae is a 100 kg (220 lb) robotic probe that set down on the surface with landing gear.[17][53] The landing site has been christened Agilkia in honour of Agilkia Island, where the temples of Philae Island were relocated after the construction of the Aswan Dam flooded the island.[54] The acceleration due to gravity on the surface of Churyumov–Gerasimenko has been estimated for simulation purposes at 10−3 m/s2,[55] or about one ten-thousandth of that on Earth.

Because of its low relative mass, landing on the comet involved certain technical considerations to keep Philae anchored. The probe contains an array of mechanisms designed to manage Churyumov–Gerasimenko's low gravity, including a cold gas thruster, harpoons, landing-leg-mounted ice screws, and a flywheel to keep it oriented during its descent.[56][57][58] During the event, the thruster and the harpoons failed to operate, and the ice screws did not gain a grip. The lander bounced twice and only came to rest when it made contact with the surface for the third time,[59] two hours after first contact.[60]

Contact with Philae was lost on 15 November 2014 because of dropping battery power. The European Space Operations Centre briefly reestablished communications on 14 June 2015 and reported a healthy spacecraft but communications were lost again soon after.[61] On 2 September 2016, Philae was located in photographs taken by the Rosetta orbiter. It had come to rest in a crack with only its body and two legs visible. While the discovery solves the question of the lander's disposition, it also allows project scientists to properly contextualise the data it returned from the comet's surface.[62]

The composition of water vapor from Churyumov–Gerasimenko, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. The ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water found on Earth came from comets such as Churyumov–Gerasimenko.[9][63][64] On 22 January 2015, NASA reported that, between June and August 2014, the comet released increasing amounts of water vapor, up to tenfold as much.[65] On 23 January 2015, the journal Science published a special issue of scientific studies related to the comet.[66]

Measurements carried out before Philae's batteries failed indicate that the dust layer could be as much as 20 cm (8 in) thick. Beneath that is hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet.[67]

The nucleus of Churyumov–Gerasimenko was found to have no magnetic field of its own after measurements were taken during Philae's descent and landing by its ROMAP instrument and Rosetta's RPC-MAG instrument. This suggests that magnetism may not have played a role in the early formation of the Solar System, as had previously been hypothesized.[68][69]

Solid organic compounds were also found in the dust particles emitted by the comet; the carbon in this organic material is bound in "very large macromolecular compounds", analogous to the insoluble organic matter in carbonaceous chondrite meteorites. Scientists think that the observed cometary carbonaceous solid matter could have the same origin as the meteoritic insoluble organic matter, but suffered less modification before or after being incorporated into the comet.[81]

One of the most outstanding discoveries of the mission so far is the detection of large amounts of free molecular oxygen (O2) gas surrounding the comet. Current solar system models suggest the molecular oxygen should have disappeared by the time 67P was created, about 4.6 billion years ago in a violent and hot process that would have caused the oxygen to react with hydrogen and form water.[82][83] Molecular oxygen has never before been detected in cometary comas. In situ measurements indicate that the O2/H2O ratio is isotropic in the coma and does not change systematically with heliocentric distance, suggesting that primordial O2 was incorporated into the nucleus during the comet's formation.[82] Detection of molecular nitrogen (N2) in the comet suggests that its cometary grains formed in low-temperature conditions below 30 K (−243 °C; −406 °F).[84]

On 3 July 2018, researchers reported that molecular oxygen is not made on the surface of comet 67P, a finding that supports the notion that the oxygen comes from the body of the comet, and may be primordial.[85][86]

CAESAR is a proposed sample-return mission aimed at returning to 67P/Churyumov–Gerasimenko, capturing regolith from the surface, and returning it to Earth.[87][88] This mission is competing in NASA's New Frontiers mission 4 selection process, and as of December 2017 is one of two finalists in the program.[89]

^O'Neill, Ian (12 November 2014). "Rosetta's Lander Grabs Onto Comet and Lands". Discovery News. Retrieved 13 November 2014. As there was a real risk of the lander bouncing off the comet, harpoons, landing leg ice screws and thrusters needed to work in concert to ensure Philae stayed in place.